Buffy did exactly this when she was about 4 months old or so. I think for her I needed to understand that she was generally not being stubborn, she was just a little hesitant and unsure of where she was going and the big noisy things everywhere.
The good news is that this is a puppy phase... Buffy got a lot less fearful after a month or so. Although we working with her to get her used to the things she was scared of, like busses, trains, etc.
Also, we did short little comes.... squat down, lots of encouragement, sometimes a food lure or a squeaky toy, and then lots of praise when she came a short distance toward me. And the sit-let's go-sit patten, with a very short "let's go" and praise or treat each time they do it right. Once Buffy got the hang of heeling she was a pro at it.
I find if you just tug they don't want to come, they just plant their feet (although I will tug if Buffy plants in the middle of a street or somewhere dangerous). The encouragement method means you look silly walking the dog for awhile, but it will pay off eventually. Doing training on walks gets the dog focused on you and really improves the heeling.
So Smudge ended up with a "people taxi" for a long time. In other words I just carried him. 
